The Green Lantern Corps has kept the DC Universe safe for millennia, and that isn't a mistake. While the Green Lanterns are far from the only wielders of light, they're the most successful organization for one reason alone.
As the oldest beings in the DCU, the Guardians of the Universe took it upon themselves to keep order across time and space. They first attempted this with their robotic army known as the Manhunters, but a programming malfunction caused them to wipe out an entire sector of space, murdering billions of innocent lives. Recognizing their army needs a personal touch, the Green Lantern Corps was formed and beings capable of understanding right and wrong were recruited. To aid their mission, they tapped into the Emotional Spectrum and used the Green Light of Willpower as the base energy to power their ultimate weapon: the Green Lantern's Power Ring.
But after millennia of the Green Lanterns being the sole light-bearers in the DCU, Hal Jordan discovered that more lights were getting ready to join the party. In Green Lantern #20 by Geoff Johns and Daniel Acuña, Hal faces the parasitic Star Sapphire, who is currently possessing his love interest, Jillian Pearlman. Carol Ferris tells Hal about the Star Sapphires and their leaders, the all-female Zamaron tribe. Green Lantern realizes that the Zamarons have harnessed another light and have made their own Corps, fueled by love instead of willpower. However, the Zamarons footsoliders, the Star Sapphires, have little control of their actions as long as they're under the influence of the Violet Light. The Zamarons even mention how dangerous their light is, noting that the lights at the far ends of the Emotional Spectrum are much more dominant than those closer to the center.
The Zamarons are not wrong in their assessment of the Emotional Spectrum. The Red Lantern Corps, the army on the opposite ends of the Spectrum, all became bloodthirsty animals after being subjected to the Red Light of Rage. Even the Orange and Indigo Lights have certain possessive abilities that rob their users autonomy and create massive personality changes. By forming a corps out of the Green Light, the Guardians of the Universe ensured that their members would never fall to the unfortunate drawbacks that come with the Spectrum's other lights. One of the biggest criticisms of the Guardians of the Universe regards their cold, unfeeling demeanor. When forming a Corps, they intentionally avoided most of the Emotional Spectrum out of the danger emotions possessed. And seeing the sorts of damage the other lights are capable of, that was probably the smart move.
The Green Light is powered by will and Lanterns are taught to overcome their other emotions in order to use it properly. They don't face the same possessive influence that hounds the Red Lanterns or Star Sapphires. The Green Lantern Corps may not wield the most exciting light, but it's kept them a premier fighting force much longer than any army in the Emotional Spectrum.